The present invention relates to a process for preparing compounds carrying at least a urethane functional group by reacting compounds carrying isocyanate functional groups and compounds carrying hydroxyl groups, this process being specifically adapted to the synthesis of polyurethanes.
The formation of a urethane functional group (divalent functional group of formula —O—C(═O)—NH—, also known as a carbamate functional group) by reacting a hydroxyl group (—OH) and an isocyanate functional group (—N═C═O) is a well-known reaction which has been the subject of numerous publications. It is generally acknowledged that it implements the following formation mechanism:

By using compounds carrying at least two isocyanate groups and compounds carrying at least two hydroxyl groups in the above reaction, polymerisation takes place which leads to polyurethanes comprising a succession of units of the type [—R—NH—CO—O—R′—O—CO—NH—].
The aforementioned reactions for forming urethane functional groups are admittedly generally, perhaps even completely, quantitative and are most often carried out at ambient temperature or above, especially when primary or secondary alcohols are used as hydroxyl compounds (and more specifically when primary alcohols are used). However, it is generally advised to catalyse these reactions, particularly in the case of reactions in which polyurethanes are formed.
Especially, the reaction is catalysed so as to minimise any side reactions which may produce unwanted by-products. In fact, the isocyanate functional group, which is very reactive, reacts very easily not only with the —OH groups, but also with all the nucleophilic species carrying labile protons present in the reaction medium. In particular, the isocyanate functional groups are likely to react with water to form an unstable carbamic acid which breaks down into amine. The amine thus formed may subsequently react with another isocyanate group so as to form a urea which, in turn, may add onto another isocyanate group to form a biuret.
In order to catalyse reactions forming urethane functional groups, and especially, polyurethane synthesis reactions, it has been suggested inter alia, to use organometallic catalysts, especially organometallic compounds based on metals such as tin, similar to those described, for example, in “Waterborne & Solvent Based Surface Coatings and their Applications”, ISBN 0471 078868, Volume III: Polyurethanes, P. Ardaud, E. Charrière-Perroud, C. Varron, edited by P. Thomas, and published in 1998 by SITA Technology Ltd., or else in the article “Structural engineering of polyurethane coatings for high performance applications” from Progress in Polymer Science, Volume 32, Issue 3, pages 352-418 D. K. Chattopadhyay and K. V. S. N. Raju (March 2007).
These catalysts based on tin-type metals have proven unsatisfactory, particularly when it is sought to catalyse a polyurethane synthesis reaction. In fact, these catalysts exhibit high toxicity which means they must be handled in a controlled manner, in particular so as to avoid any negative impact on the environment.
Furthermore, the metals of these catalysts are also likely to pollute the synthesized urethanes, which would be particularly problematic if polymer compositions are synthesized, in which case the metals tend to become trapped in a relatively irreversible manner, thus compromising numerous potential applications of the polymer compositions obtained.